14.5 Symbolic Links

GNU systems support soft links or symbolic links. This is a kind of “file” that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file.

The reason symbolic links work the way they do is that special things happen when you try to open the link. The open function realizes you have specified the name of a link, reads the file name contained in the link, and opens that file name instead. The stat function likewise operates on the file that the symbolic link points to, instead of on the link itself.

By contrast, other operations such as deleting or renaming the file operate on the link itself. The functions readlink and lstat also refrain from following symbolic links, because their purpose is to obtain information about the link. link, the function that makes a hard link, does too. It makes a hard link to the symbolic link, which one rarely wants.

Some systems have for some functions operating on files have a limit on how many symbolic links are followed when resolving a path name. The limit if it exists is published in the sys/param.h header file.

Macro: int MAXSYMLINKS

The macro MAXSYMLINKS specifies how many symlinks some function will follow before returning ELOOP. Not all functions behave the same and this value is not the same a that returned for _SC_SYMLOOP by sysconf. In fact, the sysconf result can indicate that there is no fixed limit although MAXSYMLINKS exists and has a finite value.

Prototypes for most of the functions listed in this section are in unistd.h.

Function: int symlink (const char *oldname, const char *newname)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The symlink function makes a symbolic link to oldname named newname.

The normal return value from symlink is 0. A return value of -1 indicates an error. In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EEXIST

There is already an existing file named newname.

EROFS

The file newname would exist on a read-only file system.

ENOSPC

The directory or file system cannot be extended to make the new link.

EIO

A hardware error occurred while reading or writing data on the disk.

Function: ssize_t readlink (const char *filename, char *buffer, size_t size)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The readlink function gets the value of the symbolic link filename. The file name that the link points to is copied into buffer. This file name string is not null-terminated; readlink normally returns the number of characters copied. The size argument specifies the maximum number of characters to copy, usually the allocation size of buffer.

If the return value equals size, you cannot tell whether or not there was room to return the entire name. So make a bigger buffer and call readlink again. Here is an example:

char *
readlink_malloc (const char *filename)
{
  int size = 100;
  char *buffer = NULL;

  while (1)
    {
      buffer = (char *) xrealloc (buffer, size);
      int nchars = readlink (filename, buffer, size);
      if (nchars < 0)
        {
          free (buffer);
          return NULL;
        }
      if (nchars < size)
        return buffer;
      size *= 2;
    }
}

A value of -1 is returned in case of error. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EINVAL

The named file is not a symbolic link.

EIO

A hardware error occurred while reading or writing data on the disk.

In some situations it is desirable to resolve all the symbolic links to get the real name of a file where no prefix names a symbolic link which is followed and no filename in the path is . or ... This is for instance desirable if files have to be compare in which case different names can refer to the same inode.

Function: char * canonicalize_file_name (const char *name)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | See POSIX Safety Concepts.

The canonicalize_file_name function returns the absolute name of the file named by name which contains no ., .. components nor any repeated path separators (/) or symlinks. The result is passed back as the return value of the function in a block of memory allocated with malloc. If the result is not used anymore the memory should be freed with a call to free.

If any of the path components is missing the function returns a NULL pointer. This is also what is returned if the length of the path reaches or exceeds PATH_MAX characters. In any case errno is set accordingly.

ENAMETOOLONG

The resulting path is too long. This error only occurs on systems which have a limit on the file name length.

EACCES

At least one of the path components is not readable.

ENOENT

The input file name is empty.

ENOENT

At least one of the path components does not exist.

ELOOP

More than MAXSYMLINKS many symlinks have been followed.

This function is a GNU extension and is declared in stdlib.h.

The Unix standard includes a similar function which differs from canonicalize_file_name in that the user has to provide the buffer where the result is placed in.

Function: char * realpath (const char *restrict name, char *restrict resolved)

Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | See POSIX Safety Concepts.

A call to realpath where the resolved parameter is NULL behaves exactly like canonicalize_file_name. The function allocates a buffer for the file name and returns a pointer to it. If resolved is not NULL it points to a buffer into which the result is copied. It is the callers responsibility to allocate a buffer which is large enough. On systems which define PATH_MAX this means the buffer must be large enough for a pathname of this size. For systems without limitations on the pathname length the requirement cannot be met and programs should not call realpath with anything but NULL for the second parameter.

One other difference is that the buffer resolved (if nonzero) will contain the part of the path component which does not exist or is not readable if the function returns NULL and errno is set to EACCES or ENOENT.

This function is declared in stdlib.h.

The advantage of using this function is that it is more widely available. The drawback is that it reports failures for long path on systems which have no limits on the file name length.